I think lot of habitual eaters are also doing price comparisons between different options. So just being slightly cheaper than competition can move that specific purchasing decision from one brand to an other. So also being able to nudge them specifically will not affect the other group.

I suspect there is actually three groups, because the app also has a sort of coupon-ing in it. The lowest price requires you to search the app to find a "deal" that is currently available, rather than just use your loyalty points to get a discount.

One of the ideas of classic coupons was to offer a lower price to the most price sensitive customers. The people willing to deal with the fuss of finding and cutting out the coupons.